In recent years, in response to social demands and movements arising from energy and environmental issues, fuel cells that can be operated even at normal temperature to obtain high power density are attracting attention as power sources for electric vehicles and as stationary power sources. Fuel cells are clean power generating systems having almost no adverse impact on the global environment because the product generated by an electrode reaction is water in principle. Particularly, polymer electrolyte fuel cells (PEFCs) are operated at relatively low temperature and are therefore anticipated to be used as power sources for electric vehicles. Polymer electrolyte fuel cells are generally configured to have a structure in which an electrolyte membrane-electrode assembly (MEA) is sandwiched between separators. An electrolyte membrane-electrode assembly is formed such that a polymer electrolyte membrane is interposed by a pair of an electrode catalyst layers and a pair of gas diffusion electrodes (gas diffusion layers; GDL).
In a polymer electrolyte fuel cell having an electrolyte membrane-electrode assembly such as described above, an electrode reaction represented by the reaction formula described below is caused to progress in the two electrodes (cathode and anode) that sandwich the solid polymer electrolyte membrane according to the polarities of the electrodes, and thus electric energy is obtained. First, hydrogen contained in the fuel gas supplied to the anode (negative electrode) side is oxidized by a catalyst component and produces protons and electrons (2H2→4H++4e−: Reaction 1). Next, protons thus produced pass through the solid polymer electrolyte included in the electrode catalyst layer and the solid polymer electrolyte membrane that is in contact with the electrode catalyst layer, and reach the electrode catalyst layer on the cathode (positive electrode) side. Furthermore, electrons produced in the electrode catalyst layer on the anode side pass through an electroconductive carrier that constitutes the electrode catalyst layer, a gas diffusion layer that is in contact with a side of the electrode catalyst layer, the side being different from the solid polymer electrolyte membrane, a separator, and an external circuit, and reach the electrode catalyst layer on the cathode side. The protons and electrons that have reached the electrode catalyst layer on the cathode side react with oxygen contained in an oxidizing agent gas that is supplied to the cathode side, and produce water (O2+4H++4e−→2H2O: Reaction 2). In a fuel cell, it is possible to extract electricity to the outside through the electrochemical reaction described above.
For the purpose of enhancing the power generation performance, for example, metal nanoparticles having a konpeito shape in which dendritic parts extend radially from the central part are reported in Patent Literature 1. According to Patent Literature 1, it is described that since the specific surface area of the metal nanoparticles can increase while the metal nanoparticles have a thermally stable particle size, the catalytic function can be enhanced.